In my above-identified application, there is described a CRT that uses a long essentially cylindrical envelope in which an electron beam is launched to travel longitudinally parallel to the longitudinal axis of the envelope. Extending longitudinally along a sidewall of the envelope is included a fiber optics faceplate whose inner surface is coated with a phosphor and whose outer surface is adopted to contact a photosensitive recording medium. An array of electromagnets is included along the envelope and, when appropriately energized, serves to deflect transversely the electron beam at varying points along its path such that the electron beam sweeps along the faceplate in a line scan.
In such a tube, it is particularly important to calibrate and adjust the electron beam current and beam position. Moreover, it is difficult to monitor the electron beam on the sidewall fiber optics faceplate to effect such calibration and adjustment.